The present invention relates generally to a water heating system including a valve located between the compressor outlet and the expansion device inlet to which is utilized to defrost passages in the evaporator.
Chlorine containing refrigerants have been phased out in most of the world due to their ozone destroying potential. Hydrofluoro carbons (HFCs) have been used as replacement refrigerants, but these refrigerants still have high global warming potential. “Natural” refrigerants, such as carbon dioxide and propane, have been proposed as replacement fluids. Unfortunately, there are problems with the use of many of these fluids as well. Carbon dioxide has a low critical point, which causes most air conditioning systems utilizing carbon dioxide to run partially above the critical point, or to run transcritical, under most conditions. The pressure of any subcritical fluid is a function of temperature under saturated conditions (when both liquid and vapor are present). However, when the temperature of the fluid is higher than the critical temperature (supercritical), the pressure becomes a function of the density of the fluid.
In a transcritical vapor compression system, the refrigerant is compressed to a high pressure in the compressor. As the refrigerant enters the gas cooler, heat is removed from the high pressure refrigerant. The heat is transferred to a fluid medium in a heat sink, such as water. The fluid medium is pumped through the gas cooler by a water pump. Next, after passing through an expansion device, the refrigerant is expanded to a low pressure. The refrigerant then passes through an evaporator and accepts heat from outdoor air. The refrigerant then re-enters the compressor completing the cycle.
If the surface temperature of the evaporator is below the dew-point temperature of the moist outdoor air, water droplets condense onto the evaporator fins. When the surface temperature of the evaporator is below freezing, the water droplets can freeze. Frost crystals grow from the frozen droplets and block the passage of air through the evaporator. The blockage increases the pressure drop through the evaporator, reducing the airflow through the evaporator, degrading heat pump performance, and reducing heating capacity.
In the prior art, the evaporator has been defrosted by deactivating the water pump in the gas cooler. The hot refrigerant from the compressor flows through the gas cooler without rejecting heat to the fluid in the gas cooler. The hot refrigerant is expanded and flows through the evaporator to defrost the evaporator. A drawback to this prior art system is that immediately after the water pump is deactivated, the gas cooler is still cold from the fluid. Therefore, the refrigerant must flow through the gas cooler while the water pump is off to warm the gas cooler. Once the gas cooler is warmed, the opening of the expansion device is enlarged to provide the warmed refrigerant to the evaporator. This system also incurs a greater pressure drop from the exit of the compressor to the inlet of the expansion device as the refrigerant must flow the long path through the gas cooler. This also requires that the opening degree of the expansion device be increased.
Hence, there is a need in the art for an improved defrosting methodology that overcomes these problems of the prior art.